Many desirable solar cell device structures cannot be formed using silicon wafers due to the inability to control the type and concentration of impurities more than a few microns from the surface (other than the wafer's background doping which is inevitably quite uniform throughout the wafer). Semiconductor layers formed by deposition and growth processes (such as Plasma Enhanced Chemical Vapour Deposition, Ion-assisted deposition, E-beam deposition, sputtering, solid phase crystallisation of deposited amorphous layers, liquid phase epitaxial growth etc.) provide the opportunity of controlling the impurity type and concentration throughout the thickness of the semiconductor material or layers; however these are often costly and lead to the formation of lower quality silicon. Whilst these techniques provide a method for controlling the dopant concentration and type throughout the entire thickness of the grown device (which is potentially greater than 4 um), such methods cannot be applied to conventional silicon wafers which do not undergo deposition and growth processes whereby the material can be tailored at each layer.
Several techniques have been developed for modifying the doping concentration and type within close proximity of silicon wafer surfaces, although until now, no such technique has been able to control the doping more than about 4 microns from the wafer surface. Many silicon solar cells use various thermal processes to incorporate dopants into the surface of the silicon wafer, often with junction depths of less than 1 micron. Such diffusions may be fabricated by various methods. One such method which is normally used for the fabrication of conventional screen printed solar cells is the use of thermal diffusions in furnaces. These diffusion processes typically only allow dopants to penetrate less than 200 nm deep, due to the difficulty in achieving greater depths without lengthy and excessively high temperature processing. Ion implantation is another technique used for introducing dopants into a silicon wafer. However, this causes substantial damage to the material due to the high energy bombardment of the atoms. To create deep junctions, the power must be increased which leads to further damage being caused. As a result, junction depths are typically kept to <1 um, and even then they are specially treated to minimize formation of defects.
The inability to control the type and concentration of impurities more than a few microns from the silicon wafer surface greatly limits the ability to form many device structures and in particular, many structures that would enhance the performance and durability of photovoltaic devices. In an attempt to control doping deeper within the wafer, many approaches have been developed for modifying the wafer surface such as with the formation of grooves and holes to enable the surface to penetrate to deep within the wafer (more than 4 microns from the original surface), thereby allowing dopants to enter such surfaces and effectively allow dopants to be introduced into regions deep within the wafer. Examples include the Buried Contact Solar Cell and the Emitter Wrap-Through Cell. Although many innovative device structures can be formed in this way, it does not overcome the fundamental limitation that the impurities are still within several microns of the semiconductor surface. This is an important limitation for many reasons such as when forming a heavily doped junction beneath a metal contact formed on the surface of the semiconductor. It is desirable to have such junctions as deep as possible to minimise the dark saturation current contribution from the metal/silicon interface and also to improve the durability of the device during subsequent thermal processes or during operation in the field where the greater distance from the metal to the junction makes it more difficult for the metal to penetrate to the junction and damage the device.
Laser doping can also be used to form heavily doped layers or regions. By applying heat to the desired regions, localised diffused regions are formed and diffusion can occur through either solid state or liquid state diffusion. For the laser doping process, conventional 532 nm wavelength Q-switched lasers with nanosecond pulse lengths are typically used. If performed using solid state diffusion, the junction depth will be substantially smaller due to the lower diffusion coefficients of dopants in the solid phase than that of the liquid phase. However if the diffusion is performed in the liquid phase, the pulse frequency of conventional Q-switched lasers results in the semiconductor material solidifying in between successive pulses. As a result, additional pulses merely melt the same volume of silicon and do not lead to a deeper junction. Using such lasers, to obtain a deeper junction depth, higher energy pulses must be used. However if the energy is too high, this leads to ablation of the silicon material which will result in the formation of grooves; voids and possibly even cracks, again preventing the achievement of doping of the silicon more than 4 microns from the surface. As a result, junction depths of more than 4 microns cannot be obtained using these lasers. Even when using longer wavelength Q-switched lasers (such as 1064 nm) which have a deeper absorption depth, deep junctions cannot be obtained. Although the longer wavelength light is absorbed over a larger volume of the solid silicon which could allow for larger diffusion depths, once the laser illumination melts the silicon, the absorption of infrared by the molten region is greatly enhanced which causes the ablation of the surface layer.